Hydroacoustic transducers for operation in the transmitter or receiver mode have undergone a substantial evolution. Different frequency ranges of interest, sensitivities and range capabilities as well as increasing pressures and high speed platforms have caused transducers to be made in a wide variety of designs.
Transducers having ferroelectric or magnetostrictive active elements have enabled the coupling of acoustic energy in substantial amounts through differently shaped and sized projection surfaces. Generally speaking, most transducers, particularly ferroelectric and magnetostrictive designs, have a radiation pattern that is generally considered to be omnidirectional in character. Usually this is because the dimensions of the radiating surface of the transducer are small compared to the wavelength of sound in the ambient sound-propogating medium. The wavelength of sound is generally in the low kilohertz range and below so that many if not all transducers, standing alone, have a small dimensioned radiating surface with respect to the transmitted acoustic signal. Since this is the case, it naturally follows that the radiation patterns will be omnidirectional.
A usual means for obtaining directivity with such transducers is by taking several of them and arranging them in phased arrays or providing a number of focusing lenses or reflectors. Usually, however, the arrays of transducers are themselves massive to create an even more ponderous and unmanageable apparatus. In addition, some of the equipments of the lenses or reflectors are fabricated to define cavities or compliant tubes that need pressure compensation for greater operational depths. A consequence of pressure compensation, of course, is to compromise the compliance of the lens structure so that the transducer response characteristic changes with changing depths.
A noteworthy example of an electromechanical transducer having cavities filled with a liquid is set forth in U.S. Pat. No. 3,274,537 by William J. Toulis. Hollow glass spheres are filled with air or compositions which would be characterized by compressibility that is substantially greater than the compressibility of the liquid within which they are immersed. The pressure compensation scheme of Toulis used to achieve pressure equalization relied upon a liquid filling that affect the transducer resonance. There was nothing critical about the orientation of the pressure-release-material filled compliant tubes within the transducer cavity. Resonance could be shifted in this manner with an omnidirectional response.
One way to avoid changes in resonance and response is disclosed in U.S. Pat. No. 3,375,489. Hollow steel glass or solid lead balls are placed in the inside of a transducer and function as support members to retain a space within the container that has a lower sound velocity than the sound velocity of the surrounding water medium. The omnidirectional response of this transducer is said to be substantially unchanged throughout a forseable deployment depth. Another approach was the use of microballoons formed of glass, ceramic or other non-organic materials that fill the interior of the electroacoustic transducer of R. J. Cyr in his U.S. Pat. No., 3,372,370. Pressure compensation using hollow spheres is provided for to enable a substantially uninterrupted projection irrespective of changing depths.
A baffle structure for an underwater transducer array of Frank Massa U.S. Pat. No. 3,699,507 has a number of rigid hollow capsules that are welded into rigid circular sleeves each housing a transducer. The capsules are provided to increase the front-to-back impedance to passage of sonic energy through the interstices and present a high acoustic impedance to prevent passage of interfering, out-of-phase acoustic energy from the back of the front of the array of transducer elements. The combined sonic radiation from the array of transducers produced a desired source level and directional radiation pattern. Acoustic coupling to the underwater environment without degradation by the adjacent transducer elements could be accomplished by a typical array of vibrational piston transducers since this arrangement avoided a degradation in acoustic coupling within the array that was otherwise caused by an interference between out-of-phase radiations from the front and back surfaces of vibrating transducers.
The broadband acoustic transducer of D. E. Andrews, Jr. in U.S. Pat. No. 3,302,163 had a number of very low acoustic impedance portions made up of soft pressure release materials to increase the bandwidths and directivity of an underwater acoustic transducer. The soft pressure release materials located in an outwardly flaring cone design with respect to a ferroelectric cylinder gave broadband as well as a directivity for response.
Thus there is a continuing need in the state-of-the-art for a single transducer element having a not unduly directional hemispherical response that is compact, uncomplicated and rugged in design and capable of responsive operation at changing depths.